Introduction
Quercus (oak) has high pollen frequencies in the Pleistocene and Holocene sediments in the United States. Palynologists treat the composite pollen type “Quercus ” as a high temperature indicator, while an indistinguishable blend of Pinus pollen was marked as a low temperature indicator in paleopalynological studies. In addition, researches on modern oaks suggest that there are correlations between the Quercus pollen reproduction, drought, and temperature (Adam, 1988; Mensing, 2001). A study of Quercus pollen in the Derby area, UK, suggests that the percentage of mean Quercuspollen increased while the percentage of mean average daily May temperature increased from 1970 to 1997 (Corden and Millington, 1999). Bartlein et al. (1984) found that there is a positive correlation between mean July temperature and Quercus pollen frequency in their mapping of Holocene pollen data in the midwestern United States. In numerous studies, fossil pollen frequencies of Quercus can be correlated to the paleotemperature proxy, oxygen isotope values, in global warming events (Adam et al, 1981; Adam 1985, 1988; Heusser, 2000; Heusser et al., 2000).
There are around 89 species of Quercus in the United States, and 46 are endemics (Nixon and Muller, 1997). The identification ofQuercus species mainly depends on phylogenetic and foliage morphological studies. The classification of phenotypic morphologies is constant to that of genetic morphologies of most extant oak species. However, interspecific gene flow between Quercus trees causes molecular phylogeny estimation problematic. Former comprehensive studies establish recent Quercus classification and clades (Denk and Grimm, 2009; Denk et al., 2012; Hubert et al., 2014; Manos et al., 2014; Hipp et al., 2014). California boasts around 26 oak (Quercus ) species, many of which with widespread distributions in northern and southern states. They are classified as three sections, Quercus(the white oaks), Lobatae Loudon (the red or black oaks), andProtobalanus (Trelease) A.Camus (the intermediate or golden oaks) (Hipp et al., 2014). The fossil record sets the divergence of sectionsQuercus and Lobatae at a minimum of 23–33 mya (Borgardt and Pigg, 1999; Daghlian and Crepet, 1983). The various taxa live in habitats with varying climatic features (Table 1; Species distributions information from eJepson, 2016). For instance, the distribution ofQuercus kelloggii (mean altitude of ~4750 feet) has a more elevated distribution than Quercus agrifolia (mean altitude ~830 feet). Accordingly, the northern boundary of Quercus kelloggii populations extend further north thanQuercus agrifolia . Quercus sadleriana lives at Klamath Ranges. The distribution of Quercus pacifica and Quercus tomentella is only around Channel Islands. The variety ofQuercies habitats in California indicate different limit of each species. Furthermore, the wide range of habitat preferences of California Quercus taxa implies that simply interpreting increase of Quercus pollen frequencies as “warm” may lead to wrong interpretations, or interpretations that lack the nuances of the conditions that produced the palynological assemblage. Therefore, to identify oak pollen at the species level will give pollen analysis on climate and environmental reconstruction more accurate results.
Table 1 Distribution of Quercus species in California
Scientific name Elevation Latitude Range (CA floral province)
Quercus cornelius-mulleri 300m – 2140m 32 – 43 High Sierra Nevada, San Gabriel Mountains, San Bernardino Mountains, Peninsular Ranges, s Desert Mountains, Sonoran Desert, Baja California
Quercus engelmannii 00m – 1300m 32 – 34.5 South Coast, South Channel Islands, San Gabriel Mountains, Peninsular Ranges, Baja California
Quercus john-tuckeri 900m – 2090m 33 – 36.5 Inner South Coast Ranges, Western Transverse Ranges, San Gabriel Mountains, southwest edge Mojave desert
Quercus sadleriana 600m – 2200m 41 – 42 Klamath Ranges
Quercus turbinella 1200 – 2000m 33 – 36 E. Mojave Desert; Baja California
Quercus durata 150m – 1500m 34 – 41 High North Coast Ranges, Inner North Coast Ranges, Outer North Coast Ranges, North Sierra Nevada Foothills, North High Sierra Nevada, Central Coast, Inner South Coast Ranges, Outer South Coast Ranges, Western Transverse Ranges, San Gabriel Mountains
Quercus dumosa 00m – 200m 33 – 40 South Coast, Peninsular Ranges, Baja California
Quercus pacifica 00m – 610m 33.5 – 34 Channel Islands
Quercus douglasii 00m – 1590m 33.5 – 41.5 Outer North Coast Ranges, Inner North Coast Ranges, Cascade Range Foothills, Sierra Nevada Foothills, Tehachapi Mountain Area, Sacramento Valley, San Joaquin Valley, San Francisco Bay Area, South Coast Ranges, South Channel Islands, Western Transverse Ranges, Modoc Plateau
Quercus garryana 90m – 2140m 34.5 – 42 North Coast, Klamath Ranges, High Cascade Range, Modoc Plateau, Warner Mountains, High North Coast Ranges, Inner North Coast Ranges, Outer North Coast Ranges, Cascade Range Foothills, North High Sierra Nevada, South High Sierra Nevada, Central Coast, San Francisco Bay Area, n Sierra Nevada Foothills, Central Sierra Nevada Foothills, South Sierra Nevada Foothills, Outer South Coast Ranges, Western Transverse Ranges
Quercus lobata 00m – 1830m 34 – 40 North Coast Ranges, Cascade Range Foothills, Sierra Nevada Foothills, South High Sierra Nevada, Tehachapi Mountain Area, Great Central Valley, San Francisco Bay Area, South Coast Ranges, NW South Coast, Channel Islands (Santa Cruz, Santa Catalina islands), Western Transverse Ranges, w San Gabriel Mountains
Quercus chrysolepis var. nana 30m – 2750m 33 – 42 North Coast, Klamath Ranges, High Cascade Range, Cascade Range Foothills, North High Sierra Nevada, Central High Sierra Nevada, South High Sierra Nevada, North Sierra Nevada Foothills, Central Sierra Nevada Foothills, South Sierra Nevada Foothills, Desert Mountains, Central Coast, San Francisco Bay Area, Inner South Coast Ranges, Outer South Coast Ranges, South Coast, Western Transverse Ranges, San Gabriel Mountains, San Bernardino Mountains, Peninsular Ranges, San Jacinto Mountains
Quercus palmeri 300m – 1600m 33 – 36 East Inner North Coast Ranges (Colusa Co.), South High Sierra Nevada (East slope), Northwest San Joaquin Valley (Alameda, Contra Costa cos.), San Francisco Bay Area (Alameda Co.), South Coast Ranges, Western Transverse Ranges, San Gabriel Mountains (North slope), San Bernardino Mountains (North slope), East Peninsular Ranges, South edge Mojave Desert, Desert Mountains (Little San Bernardino Mountains)
Quercus tomentella 00m – 600m 33 – 34 Channel Islands
Quercus vacciniifolia 150m – 2930m 37 – 42 Klamath Ranges, High North Coast Ranges, Inner North Coast Ranges, High Cascade Range, High Sierra Nevada, Modoc Plateau
Quercus cedrosensis 100m – 1800m 32.5 – 33 Southwest Peninsular Ranges (Otay Mountain, San Diego Co.)
Quercus agrifolia 00m – 1500m 32.5 – 39 High North Cast Ranges, Inner North Coast Ranges, Sacramento Valley, Central Coast, San Francisco Bay Area, Inner South Coast Ranges, Outer South Coast Ranges, South Coast, Western Transverse Ranges, San Gabriel Mountains, San Bernardino Mountains, Peninsular Ranges
Quercus parvula var. parvula 00m – 500m 34 – 35 Outer South Coast Ranges (Santa Barbara Co.), n Channel Islands (Santa Cruz Island), e Western Transverse Ranges
Quercus wislizeni 00m – 2000m 33 – 41 Klamath Ranges, High Cascade Range, High North Coast Ranges, Inner North Coast Ranges, Outer North Coast Ranges, Sacramento Valley, Cascade Range Foothills, High Cascade Range, n Sierra Nevada Foothills, c Sierra Nevada Foothills, s Sierra Nevada Foothills, N High Sierra Nevada, c High Sierra Nevada, s High Sierra Nevada, East of Sierra Nevada, White and Inyo Mountains, San Joaquin Valley, San Francisco Bay Area, Inner South Coast Ranges, Outer South Coast Ranges, South Coast, Western Transverse Ranges, San Gabriel Mountains, San Bernardino Mountains, Peninsular Ranges, San Jacinto Mountains
Quercus wislizeni var. frutescens 90m – 2000m 32.5 – 41 Klamath Ranges, North Coast Ranges, High Cascade Range, High Sierra Nevada, Tehachapi Mountain Area, Sacramento Valley (Sutter Buttes), San Joaquin Valley, San Francisco Bay Area, South Coast Ranges, Southwestern California (except Channel Islands)
Quercus kelloggii 30m – 2660m 33 – 42 North Coast, Klamath Ranges, High Cascade Range, Modoc Plateau, Warner Mountains, Cascade Range Foothills, North High Sierra Nevada, Central High Sierra Nevada, South High Sierra Nevada, North Sierra Nevada Foothills, Central Sierra Nevada Foothills, South Sierra Nevada Foothills, High North Coast Ranges, Inner North Coast Ranges, Outer North Coast Ranges, Central Coast, San Francisco Bay Area, Inner South Coast Ranges, Outer South Coast Ranges Western Transverse Ranges, San Gabriel Mountains, San Bernardino Mountains, Peninsular Ranges, San Jacinto Mountains
Quercus X alvordiana 180m –1300m N/A Tehachapi Mountain Area, South Coast Ranges, Western Transverse Ranges
Quercus X macdonaldii 00m – 600m N/A Channel Islands (Santa Cruz, Santa Rosa, Santa Catalina islands)
Chrysolepis chrysophylla 00m – 2000m 35 – 42 North Coast, Klamath Ranges, High Cascade Range, Cascade Range Foothills, North High Sierra Nevada, North Coast Ranges, Central Coast, San Francisco Bay Area, Outer South Coast Ranges
Notholithocarpus densiflorus var. echinoides 600m – 2000 m 40 – 42 Klamath Ranges, Cascade Range, Sierra Nevada Foothills, High Sierra Nevada